Dual-Band Omnidirectional Circularly Polarized Antenna Using Zeroth- and First-Order Modes

This letter discusses the development of a dual-band omnidirectional circularly polarized (CP) antenna using the zeroth- and the first-order resonance modes of epsilon-negative (ENG) transmission lines (TLs). The antenna is based on a circular mushroom structure with curved branches and designed without additional feeding parts for a 90 ° phase difference between two orthogonal modes as well as additional radiators for dual-band operations. A left-hand CP (LHCP) and a right-hand CP (RHCP) are obtained at the zeroth- and the first-order resonance modes, respectively, because the current direction of the curved branch, which is related to a horizontal polarization, at the zeroth-order resonance (ZOR) mode is opposite to that at the first-order resonance (FOR) mode. The measured average axial ratios at the resonance modes in the azimuthal plane are 1.57 and 0.86 dB, respectively. The measured average LHCP and RHCP gains are - 0.24 dBic at the ZOR mode and -0.51 dBic at the FOR mode, respectively. These results are congruent with simulated data.